Best known for cross-linking cytoplasmic actin into dynamic scaffolds to control cell motility, filamins are large cytoplasmic proteins increasingly found to regulate cell signaling by interacting with over 30 different receptors and signaling molecules (Feng et al., 2004 Nat Cell Biol 6:1034-1038; Stossel et al., 2001 Nature 2:138-145), including the mu opioid receptor (MOR) (Onoprishvili et al, 2003 Mol Pharmacol 64:1092-1100). Filamins are dimerized through the last carboxy-terminal repeat near the transmembrane regions, allowing an intracellular V-shaped structure that is critical for function. There are three mammalian isoforms: filamin A (FLNA), B and C.
FLNA controls cell motility by controlling the cycle of actin polymerization and depolymerization, allowing cells to move and to migrate. As actin depolymerization is linked to the inflammatory response, binding to FLNA suppresses inflammation by slowing actin polymerization and cell motility. Femtomolar naloxone and its inactive isomer, both known to bind FLNA (Wang et al., 2008 PLoS One 3:e1554), have been shown to reduce the microglial inflammatory response; i.e., pro-inflammatory factors and reactive oxygen species, of lipopolysaccaride-activated microglial cells (Liu et al, 2000 JPET 293:607-617; Qin et al., 2005 FASEB J 19:550-557). The glial inflammatory response has been implicated in neuropathic pain (Hutchinson et al., 2008 Eur J Neurosci 28:20-29) as well as the inflammatory neurotoxicity of neurodegenerative disease (Liu et al., 2003 JPET 304:1-7).
A second function of binding to FLNA is a beneficial regulation of opioid receptor signaling; i.e., a maintenance of coupling to Gi and Go proteins. MOR preferentially couples to pertussis toxin-sensitive G proteins, Gi/o (inhibitory/other), and inhibits the adenylyl cyclase/cAMP pathway (Laugwitz et al., 1993 Neuron 10:233-242; Connor et al., 1999 Clin Exp Pharmacol Physiol 26:493-499). Analgesia results from these MOR-linked inhibitory G protein (Gi/o) signaling cascades and related ion channel interactions that suppress cellular activities by hyperpolarization.
Adaptive responses of opioid receptors contribute to the development of analgesic tolerance and physical dependence, and possibly also to components of opioid addiction. A critical adaptive response of the MOR is a switch in G protein coupling from its native Gi/o proteins to stimulatory Gs proteins, resulting in opposite effects on the cell upon activation as well as analgesic tolerance and physical dependence (Wang et al., 2005 Neuroscience 135:247-261). Prevention of this G protein coupling switch by agents that bind filamin A (Wang et al, 2008 PLoS One 3:e1554), a scaffolding protein known to interact with MOR, can alleviate unwanted adaptive responses to continued opioid administration.
A chronic opioid-induced switch to Gs coupling by MOR can cause excitatory signaling, by activation of adenylyl cyclase, in place of the usual inhibitory signaling or inhibition of adenylyl cyclase (Crain et al., 1992 Brain Res 575:13-24; Crain et al., 2000 Pain 84:121-131; Gintzler et al., 2001 Mol Neurobiol 21:21-33; Wang et al., 2005 Neuroscience 135:247-261). This switch in G protein coupling from Gi/o to Gs (Wang et al., 2005 Neuroscience 135:247-261; Chakrabarti et al., 2005 Mol Brain Res 135:217-224) may be a result of the decreased efficiency of coupling to the native G proteins, the usual index of desensitization (Sim et al., 1996 J Neurosci 16:2684-2692) and still commonly considered the reason for analgesic tolerance.
The chronic opioid-induced MOR-G protein coupling switch is accompanied by stimulation of adenylyl cyclase II and IV by MOR-associated Gβγ dimers (Chakrabarti et al., 1998 Mol Pharmacol 54:655-662; Wang et al., 2005 Neuroscience 135:247-261). The interaction of the Gβγ dimer with adenylyl cyclase had previously been postulated to be the sole signaling change underlying the excitatory effects of opiates (Gintzler et al., 2001 Mol Neurobiol 21:21-33). It has further been shown that the Gβγ that interacts with adenylyl cyclases originates from the Gs protein coupling to MOR and not from the Gi/o proteins native to MOR (Wang et al., 2006 J Neurobiol 66:1302-1310).
Thus, MORs are normally inhibitory G protein-coupled receptors that couple to Gi or Go proteins to inhibit adenylyl cyclase and decrease production of the second messenger cAMP, as well as to suppress cellular activities via ion channel-mediated hyperpolarization. Opioid analgesic tolerance and dependence are also associated with that switch in G protein coupling by MOR from Gi/o to Gs (Wang et al., 2005 Neuroscience 135:247-261). This switch results in activation of adenylyl cyclase that provides essentially opposite, stimulatory, effects on the cell.
Controlling this switch in G protein coupling by MOR is the scaffolding protein FLNA, and compounds that bind a particular segment of FLNA with high affinity, like naloxone (NLX) and naltrexone (NTX), can prevent this switch (Wang et al, 2008 PLoS One 3:e1554) and the associated analgesic tolerance and dependence (Wang et al., 2005 Neuroscience 135:247-261). This switch in G protein coupling also occurs acutely, though transiently, and is potentially linked to the acute rewarding or addictive effects of opioid drugs, through CREB activation as a result of increased cAMP accumulation (Wang et al., 2009 PLoS ONE 4 (1):e4282).
Ultra-low-dose NLX or NTX have been shown to enhance opioid analgesia, minimize opioid tolerance and dependence (Crain et al., 1995 Proc Natl Acad Sci USA 92:10540-10544; Powell et al. 2002. JPET 300:588-596), as well as to attenuate the addictive properties of opioids (Leri et al., 2005 Pharmacol Biochem Behav 82:252-262; Olmstead et al., 2005 Psychopharmacology 181:576-581). An ultra-low dose of opioid antagonist was an amount initially based on in vitro studies of nociceptive dorsal root ganglion neurons and on in vivo mouse studies. It has long been hypothesized that ultra-low-dose opioid antagonists enhance analgesia and alleviate tolerance/dependence by blocking the excitatory signaling opioid receptors that underlie opioid tolerance and hyperalgesia (Crain et al., 2000 Pain 84:121-131). Later research has shown that the attenuation of opioid analgesic tolerance, dependence and addictive properties by ultra-low-dose, defined herein, naloxone or naltrexone, occurs by preventing the MOR-Gs coupling that results from chronic opiate administration (Wang et al., 2005 Neuroscience 135:247-261), and that the prevention of MOR-Gs coupling is a result of NLX or NTX binding to filamin A at approximately 4 picomolar affinity (Wang et al, 2008 PLoS One 3:e1554).
Found in all cells of the brain, CREB is a transcription factor implicated in addiction as well as learning and memory and several other experience-dependent, adaptive (or maladaptive) behaviors (Carlezon et al., 2005 Trends Neurosci 28:436-445). In general, CREB is inhibited by acute opioid treatment, an effect that is completely attenuated by chronic opioid treatment, and activated during opioid withdrawal (Guitart et al., 1992 J Neurochem 58:1168-1171). However, a regional mapping study showed that opioid withdrawal activates CREB in locus coeruleus, nucleus accumbens and amygdala but inhibits CREB in lateral ventral tegemental area and dorsal raphe nucleus (Shaw-Luthman et al., 2002 J Neurosci 22:3663-3672).
In the striatum, CREB activation has been viewed as a homeostatic adaptation, attenuating the acute rewarding effects of drugs (Nestler, 2001 Am J Addict 10:201-217; Nestler, 2004 Neuropharmacology 47:24-32). This view is supported by nucleus accumbens overexpression of CREB or a dominant-negative mutant respectively reducing or increasing the rewarding effects of opioids in the conditioned place preference test (Barot et al., 2002 Proc Natl Acad Sci USA 99:11435-11440). In conflict with this view, however, is the finding that reducing nucleus accumbens CREB via antisense attenuated cocaine reinforcement as assessed in self-administration (Choi et al., 2006 Neuroscience 137:373-383). Clearly, CREB activation is implicated in addiction, but whether it directly contributes to the acute rewarding effects of drugs or initiates a homeostatic regulation thereof appears less clear.
The several-fold increase in pS133CREB reported by Wang et al., 2009 PLoS ONE 4 (1):e4282 following acute, high-dose morphine may indicate acute dependence rather than acute rewarding effects. However, the transient nature of the MOR-Gs coupling correlating with this CREB activation suggests otherwise. In fact, the correlation of pS133CREB with the Gs coupling by MOR following this acute high-dose morphine exposure, as well as the similar treatment effects on both, suggest that this alternative signaling mode of MOR can contribute to the acute rewarding or addictive effects of opioids. This counterintuitive notion can explain the apparent paradox that ultra-low-dose NTX, while enhancing the analgesic effects of opioids, decreases the acute rewarding or addictive properties of morphine or oxycodone as measured in conditioned place preference or self-administration and reinstatement paradigms.
In considering analgesic tolerance, opioid dependence, and opioid addiction together as adaptive regulations to continued opioid exposure, a treatment that prevents MOR's signaling adaptation of switching its G protein partner can logically attenuate these seemingly divergent behavioral consequences of chronic opioid exposure.
Even though ultra-low-dose NTX blocks the conditioned place preference to oxycodone or morphine (Olmstead et al., 2005 Psychopharmacology 181:576-581), its co-self-administration only reduces the rewarding potency of these opioids but does not abolish self-administration outright (Leri et al., 2005 Pharmacol Biochem Behav 82:252-262). It is possible that a direct stimulatory effect on VTA neurons, as opposed to the proposed disinhibition via inhibition of GABA interneurons (Spanagel et al., 1993 Proc Natl Acad Sci USA 89:2046-2050), can play some role in opioid reward. A MOR-Gs coupling mediation of reward, increasing with increasing drug exposure, is in keeping with current theories that the escalation of drug use signifying drug dependence can not indicate a “tolerance” to rewarding effects but instead a sensitization to rewarding effects (Zernig et al., 2007 Pharmacology 80:65-119).
The results reported in Wang et al., 2009 PLoS ONE 4 (1):e4282 demonstrated that acute, high-dose morphine causes an immediate but transient switch in G protein coupling by MOR from Go to Gs similar to the persistent switch caused by chronic morphine. Ultra-low-dose NLX or NTX prevented this switch and attenuated the chronic morphine-induced coupling switch by MOR. The transient nature of this acute altered coupling suggests the receptor eventually recovers and couples to its native G protein.
With chronic opioid exposure, the receptor can lose the ability to recover and continue to couple to Gs, activating the adenylyl cyclase/cAMP pathway, upregulating protein kinase A, and phosphorylating CREB as one downstream effector example. The persistently elevated phosphorylated CREB can then shape the expression of responsive genes including those closely related to drug addiction and tolerance. Importantly, the equivalent blockade of Gs coupling and pS133CREB by the pentapeptide binding site of naloxone (NLX) and naltrexone (NTX) on FLNA further elucidates the mechanism of action of ultra-low-dose NLX and NTX in their varied effects.
These data further strengthen the regulation of MOR-Gs coupling by FLNA and that binding to FLNA or using a FLNA peptide decoy for MOR can prevent the altered MOR coupling, thereby attenuating tolerance, dependence and addictive properties associated with opioid drugs.
The combination of ultra-low-dose opioid antagonists with opioid agonists formulated together in one medication has been shown to alleviate many of these undesirable aspects of opioid therapy (Burns, 2005 Recent Developments in Pain Research 115-136, ISBN: 81-308-0012-8). This approach shows promise for an improvement in analgesic efficacy, and animal data suggests reduced addictive potential. The identification of the cellular target of ultra-low-dose NLX or NTX in their inhibition of mu opioid receptor-Gs coupling as a pentapeptide segment of filamin A (Wang et al., 2008 PLoS ONE 3 (2):e1554) has led to development of assays to screen against this target to create a new generation of pain therapeutics that can provide long-lasting analgesia with minimal tolerance, dependence and addictive properties. Importantly, the non-opioid cellular target of ultra-low-dose NLX or NTX, FLNA, provides potential for developing either a therapeutic combination of which one component is not required to be ultra-low-dose, or a single-entity novel analgesic.
The present invention identifies a compound that binds to filamin A (FLNA; the high-affinity binding site of naloxone [NLX] and naltrexone [NTX]), to reduce cell motility and inflammation as well as to prevent the Gi/o-to-Gs coupling switch of MOR and is similar to or more active than DAMGO in activating MOR.